I have recently returned to this group and saw this thread so Im adding my 2 cents worth.
It is a fairly well accepted notion that using a water to antifreeze ratio of more than 50/50 gains you very little.   The heat dissipation capability of stronger solutions is impaired and the freeze/boil capabilities are no better.
The construction industry and railroad industry using mostly diesel engines recommends this mix.
As a fellow VW Westfalia owner, I would recommend using the VW product named in the owners manual, being phosphate free I believe, at a ratio of 50/50 and change it yearly to be conservative.    After all it is cheap compared to having to remove and replace the cylinder heads.
My own '84 model(67k miles)has no leaks and as far as I know has not had anything but Vw autobahn antifreeze in it.
Steve Cotsford